Beauty may or may not be only skin deep, but the skin is the part of us that other people see. Inside, we may all have the same basic assortment of organs, but it is our outer appearance that makes us look unique. Even the skin on our fingers carries a set of fingerprints that no one else has.
The skin reveals a lot about our inner selves. It grows pale when we’re afraid and flushes when we’re excited or embarrassed. It always gives us away when we’re tired. Doctors can be alerted to possible internal diseases, such as diabetes and liver disease, just by examining the skin.
The Wall Around Us
The skin is a protective shield around the body, keeping out harmful things such as bacteria and pollution. It’s naturally waterproof and will not allow water past its protective shield even when submerged for long periods of time. Skin helps to give the body shape and form and keeps important fluids and our internal organs inside and safe from harm. It protects the body from heat and cold by regulating the body temperature. Unlike a coat made of cloth, which no longer fits when you outgrow it, the skin can contract and expand like a balloon. Think of all the times you ate just a little too much, and the skin on your belly stretched, only to return later to its normal size. The skin grows around us as we grow taller and wider. It also has an amazing ability to repair itself if it is cut, torn, or burned.
Our skin is more than just a self-repairing, protective overcoat, though. It is an organ of the body, and a very active one, at that. It helps to rid the body of excess fluids, salts, and wastes. Some skin cells are like miniature factories, producing a variety of hormones and other substances and carrying out various chemical reactions. (Under the action of sunlight on the skin, for example, vitamin D is converted to an active form that helps to control the formation of bone.) The skin is also a sense organ that brings us information about the world: Through it we feel pain and pleasure, heat and cold.
It may seem a bit strange to think of the skin as an organ, like the heart or brain. What is even more surprising is that it is the largest organ in the body. If all of a person’s skin were stretched out, it would be large enough to cover a tabletop or a door—twenty square feet or so—and it weighs about eight pounds. The skin is made up of billions of cells, each microscopic in size. These cells are constantly being replaced. A skin cell lasts less than a month from the time it is formed until it falls off and becomes a tiny flake of dust. That means that every month you have a completely new skin covering on your body.
The skin is divided into three basic layers: the epidermis, the dermis, and the subcutaneous layer.
The Outer Layer
Did you know that every skin cell that you can see is dead? No, you’re not suffering from some mysterious disease. This is a perfectly normal state of affairs for the amazing outer covering of your body. The outermost layer of the skin is called the epidermis. (Epimeans “over”; it lies over the dermis, the layer that makes up the bulk of the skin.) The epidermis is about as thick as a sheet of paper. It is composed of fifteen to twenty layers of cells stacked on top of each other.
New cells are created in the deepest part of the epidermis, called the basal layer. Cells reproduce by dividing. As the skin cells get older they are pushed upward toward the outer surface of the epidermis by new cells that are forming. As they move outward, they become flatter, accumulate a horny protein called keratin, and start to lose precious moisture that keeps them alive. Eventually, by the time the epidermal cells reach the outer surface, they are completely flat and have lost most of the cell fluid. The nucleus—the ”brain” that holds the instructions for all the living cell’s activities — has disappeared.
It takes an epidermal cell about two weeks to reach the upper layer on the outer surface of the body. This layer is exposed to the air, which would quickly dry and kill living cells. But the cells of this outermost layer are all dead, and they form a hardened protective shell called the stratum corneum (literally, the “horny layer”) over the living cells beneath. At some time during the next two weeks the dead cell falls off or is washed away—perhaps speeded in its departure by the rub of a towel or the scratch of a fingernail. The loss of dead cells from the outer layer of the skin is a natural process that goes on continually. Normally we don’t notice it. But when dead skin cells are shed too quickly, psoriasis or dandruff may be the result.
The epidermis serves mostly as protection for the body. In addition to providing a physical shield, this part of the skin also protects us from the damaging ultraviolet rays of the sun. The basal layer of the epidermis contains melanocytes, cells that give the skin its color by producing tiny particles of the pigment melanin. Melanin particles absorb the sun’s harmful ultraviolet radiation and keep it from damaging the body’s delicate cells and chemicals.
A fascinating thing about melanocytes is that everyone has basically the same number of them. The reason there are people of many different colors in the world is that each person inherits genes that tell the melanocytes just how much melanin to produce and how big each melanin particle will be.
When skin is exposed to the sun, the melanocytes produce more melanin. This extra pigment produces a darkening effect—a suntan—and provides better protection. People who are very fair may not be able to make enough melanin to protect themselves effectively. Then the ultraviolet in the sun’s rays can damage skin cells, producing a painful sunburn.
The Living Skin
Beneath the epidermis is another major skin layer, the dermis. Its name comes from the Greek word for skin, and this layer makes up about 90 percent of the skin. All of the cells in the dermis are alive, and they are nourished by a rich network of millions of tiny blood vessels, called capillaries. The dermis also contains numerous nerve endings, as well as hair follicles, sweat glands, and oil glands. They are all held in place by a strong yet very elastic substance called collagen, which has been called ”nature’s nylon.”
If harmful microbes get into the dermis—perhaps through a cut or scrape in the protective barrier of the epidermis—they may begin to multiply, producing an infection. Then another series of body defenses goes into action. Chemicals released by the damaged cells cause the walls of the tiny blood vessels in the skin to get leaky, and fluid from the blood seeps out into the tissues. This process is called inflammation. Disease-fighting white blood cells squeeze their way out of the capillaries and roam through the dermis on search and destroy missions against the invading microbes.
The dermis gives the skin its strength, yet can stretch to allow the body to move freely. (As we age, the dermis grows thinner, and the accumulated effects of the years and exposure to the sun make it lose its elastic springiness; gradually it stretches out and sags, producing wrinkles.)
This important skin layer also contains a number of sensory nerves that end in specialized sense receptors. Some respond to heat, others to cold. Some are sensitive to the slightest touch, some to a firmer pressure; still others provide warning signals of pain when skin cells are being damaged.
The network of tiny blood vessels crisscrossing the dermis brings oxygen and nutrients to keep the epidermal cells alive and carries away their waste products. The capillaries also expand and contract to help cool the body down or warm it up. When the air around you is hot, special sensors in the skin send messages along nerves to the brain. These messages spark a new set of signals, which cause the capillaries to expand. Then they have a larger surface for radiating heat, which passes out through the skin directly to the air or is taken up by the water in sweat, which carries heat out of the body. Cold air against the skin surface is also reported by skin sensors, prompting signals that cause the capillaries to contract. That decreases their radiating surface and helps to conserve body heat that would otherwise be lost through the skin. The body’s system of internal ”thermostats” fine-tunes the various processes of conserving and releasing heat, so that the inner body temperature stays at about the same level no matter how hot or cold it is outside.
When the capillaries in the dermis expand, the reflections of the red blood cells that they carry give the skin a rosy color. (Not only heat but also emotions can spark this reaction. That is what happens when you blush in embarrassment.) When the capillaries contract, they take up a smaller fraction of the skin area, so not as much of the red color is visible. Then the skin turns pale. (This reaction, too, can be produced by strong emotions.)
The tiny coiled tubes of the sweat glands are found in the dermis but extend up through the epidermis and end at the surface in openings called pores. Sweat pores are too small to see with the naked eye. Over the entire body surface, there are between two and five million of these sweat glands!